package java.util.concurrent;

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

import java.util.Random;

/**
 * A random number generator isolated to the current thread. Like the global
 * {@link Random} generator used by the {@link Math} class,
 * a {@code ThreadLocalRandom} is initialized with an internally generated seed
 * that may not otherwise be modified. When applicable, use of
 * {@code ThreadLocalRandom} rather than shared {@code Random} objects in
 * concurrent programs will typically encounter much less overhead and
 * contention. Use of {@code ThreadLocalRandom} is particularly appropriate when
 * multiple tasks (for example, each a {@link ForkJoinTask}) use random numbers
 * in parallel in thread pools.
 * <p>
 * Usages of this class should typically be of the form:
 * {@code ThreadLocalRandom.current().nextX(...)} (where {@code X} is
 * {@code Int}, {@code Long}, etc). When all usages are of this form, it is
 * never possible to accidently share a {@code ThreadLocalRandom} across
 * multiple threads.
 * <p>
 * This class also provides additional commonly used bounded random generation
 * methods.
 * 
 * @since 1.7
 * @author Doug Lea
 */
// 来自JDK7 ,为了在jdk6中使用而引入
public class ThreadLocalRandom extends Random {
    // same constants as Random, but must be redeclared because private
    private static final long                           multiplier  = 0x5DEECE66DL;
    private static final long                           addend      = 0xBL;
    private static final long                           mask        = (1L << 48) - 1;

    /**
     * The random seed. We can't use super.seed.
     */
    private long                                        rnd;

    /**
     * Initialization flag to permit calls to setSeed to succeed only while
     * executing the Random constructor. We can't allow others since it would
     * cause setting seed in one part of a program to unintentionally impact
     * other usages by the thread.
     */
    boolean                                             initialized;

    /**
     * The actual ThreadLocal
     */
    private static final ThreadLocal<ThreadLocalRandom> localRandom = new ThreadLocal<ThreadLocalRandom>() {
                                                                        protected ThreadLocalRandom initialValue() {
                                                                            return new ThreadLocalRandom();
                                                                        }
                                                                    };


    /**
     * Constructor called only by localRandom.initialValue.
     */
    ThreadLocalRandom() {
        super();
        initialized = true;
    }


    /**
     * Returns the current thread's {@code ThreadLocalRandom}.
     * 
     * @return the current thread's {@code ThreadLocalRandom}
     */
    public static ThreadLocalRandom current() {
        return localRandom.get();
    }


    /**
     * Throws {@code UnsupportedOperationException}. Setting seeds in this
     * generator is not supported.
     * 
     * @throws UnsupportedOperationException always
     */
    public void setSeed(long seed) {
        if (initialized)
            throw new UnsupportedOperationException();
        rnd = (seed ^ multiplier) & mask;
    }


    protected int next(int bits) {
        rnd = (rnd * multiplier + addend) & mask;
        return (int) (rnd >>> (48 - bits));
    }


    /**
     * Returns a pseudorandom, uniformly distributed value between the given
     * least value (inclusive) and bound (exclusive).
     * 
     * @param least the least value returned
     * @param bound the upper bound (exclusive)
     * @throws IllegalArgumentException if least greater than or equal to bound
     * @return the next value
     */
    public int nextInt(int least, int bound) {
        if (least >= bound)
            throw new IllegalArgumentException();
        return nextInt(bound - least) + least;
    }


    /**
     * Returns a pseudorandom, uniformly distributed value between 0 (inclusive)
     * and the specified value (exclusive).
     * 
     * @param n the bound on the random number to be returned. Must be positive.
     * @return the next value
     * @throws IllegalArgumentException if n is not positive
     */
    public long nextLong(long n) {
        if (n <= 0)
            throw new IllegalArgumentException("n must be positive");
        // Divide n by two until small enough for nextInt. On each
        // iteration (at most 31 of them but usually much less),
        // randomly choose both whether to include high bit in result
        // (offset) and whether to continue with the lower vs upper
        // half (which makes a difference only if odd).
        long offset = 0;
        while (n >= Integer.MAX_VALUE) {
            int bits = next(2);
            long half = n >>> 1;
            long nextn = ((bits & 2) == 0) ? half : n - half;
            if ((bits & 1) == 0)
                offset += n - nextn;
            n = nextn;
        }
        return offset + nextInt((int) n);
    }


    /**
     * Returns a pseudorandom, uniformly distributed value between the given
     * least value (inclusive) and bound (exclusive).
     * 
     * @param least the least value returned
     * @param bound the upper bound (exclusive)
     * @return the next value
     * @throws IllegalArgumentException if least greater than or equal to bound
     */
    public long nextLong(long least, long bound) {
        if (least >= bound)
            throw new IllegalArgumentException();
        return nextLong(bound - least) + least;
    }


    /**
     * Returns a pseudorandom, uniformly distributed {@code double} value
     * between 0 (inclusive) and the specified value (exclusive).
     * 
     * @param n the bound on the random number to be returned. Must be positive.
     * @return the next value
     * @throws IllegalArgumentException if n is not positive
     */
    public double nextDouble(double n) {
        if (n <= 0)
            throw new IllegalArgumentException("n must be positive");
        return nextDouble() * n;
    }


    /**
     * Returns a pseudorandom, uniformly distributed value between the given
     * least value (inclusive) and bound (exclusive).
     * 
     * @param least the least value returned
     * @param bound the upper bound (exclusive)
     * @return the next value
     * @throws IllegalArgumentException if least greater than or equal to bound
     */
    public double nextDouble(double least, double bound) {
        if (least >= bound)
            throw new IllegalArgumentException();
        return nextDouble() * (bound - least) + least;
    }

    private static final long serialVersionUID = -5851777807851030925L;
}
